Folds of skin seal the eyes and ear openings when the animal dives, and the nostrils close, so vision, hearing and smell are all unavailable during a foraging dive. The bill takes over. It is studded with roughly forty thousand electroreceptors, arranged in stripes, together with a comparable number of mechanoreceptors that sense water movement and touch.
Combining the two gives a bearing and a distance. An electrical signal from a shrimp's flexing muscle arrives essentially instantly, while the pressure wave from the same movement arrives a moment later; the delay between the two encodes range. The platypus sweeps its bill side to side across the bottom while making short dives, typically well under a minute, and stores what it catches in cheek pouches to chew at the surface with horny grinding plates rather than teeth.
Adults have no true teeth at all. The often-drawn parallel with a duck's bill also misleads: the platypus bill is soft, flexible and richly innervated, nothing like a bird's keratinous beak.